GetHashFromFile.cs

public string  GetHashFromFile(string localDoArquivo)
{
    using (HashAlgorithm hashAlgorithm = HashAlgorithm.Create("SHA1"))
    {
        using (Stream stream = new FileStream(localDoArquivo, FileMode.Open, FileAccess.Read))
        {
            byte[] hash = hashAlgorithm.ComputeHash(stream);
            return BitConverter.ToString(hash);
        }
    }
}